Unlawful death cases involving 35 alleged killings have already been referred to the Service Prosecuting Authority (SPA) - the military equivalent of the Crown Prosecution Service - along with 36 cases of alleged abuse and mistreatment with "multiple complainants".

The SPA said it was also preparing to advise on an additional 20 cases of unlawful killing and 71 cases of mistreatment in the near future.

Andrew Cayley QC, the director of the SPA, said it "will not flinch" in prosecuting British soldiers where there is evidence of wrongdoing.

The former war crimes prosecutor said: "I have spent the last 20 years of my professional life advising and prosecuting in cases involving genocide, crimes against humanity and war crimes.

"I know very well what these crimes look like. Make no mistake we will give all these Ihat cases the thorough scrutiny the law requires and if prosecution is warranted we will not flinch from proceeding.

"Equally I want to make it absolutely clear that no member of the British Armed Forces will be prosecuted unless there is sufficient evidence to do so."

UK forces withdrew from Iraq in 2009 although lawyers are continuing to refer cases to the Ihat, the Government-established criminal investigation into murder, abuse and torture claims linked to the six-year military mission.

The multimillion-pound inquiry's workload reached 1,515 possible victims by September, of whom 280 are alleged to have been unlawfully killed. Ihat's budget is set at £57.2 million, which runs until the end of 2019 - 16 years after the 2003 invasion began.

An Ihat spokeswoman confirmed that some of the letters sent to veterans in the last two years had been hand delivered by detectives and that there was "no obligation to respond".

"It is standard police practice to send letters as a means of contacting potential witnesses," she said. "Sometimes the letters are delivered by hand and it may be that if a potential witness is at home then the investigator will take the opportunity to ask a few questions."

A Ministry of Defence spokeswoman said: "The vast majority of UK service personnel deployed on military operations conduct themselves professionally and in accordance with the law.

"The MoD takes all allegations of abuse or unlawful killing extremely seriously. That is why we are ensuring that they are investigated to establish the facts."

Retired Colonel Richard Kemp told the Daily Mail: "The idea that such a large number of soldiers should be accused of crime is a disgrace, and the Government should stop it.

"These soldiers did not sign up to go and put their lives on the line only to spend what could be the rest of their lives being hounded for their loyalty."

But Public Interest Lawyers, a firm that has acted in cases concerning Iraq, said Ihat and the SPA had to show they were "willing to prosecute culpable individuals".

"The Ihat was established in 2010 and some five years on there has not been a single prosecution as a result of its work," a spokesman said.

"This raises questions as to its effectiveness and its ability to deliver results for victims and relatives.

"It is important to note that the context of these allegations is the invasion and occupation of Iraq, not armed combat. The majority of the allegations of ill-treatment are in circumstances where individuals were being arrested or were in detention. There is a heavy responsibility to treat all detained persons humanely."

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